JERKY HOLOCENE SEA-LEVEL RISE ON THE WEST COAST OF CENTRAL NORTH AMERICA—AN ARCHIVE OF GREAT CASCADIA EARTHQUAKES

Upward jerks of relative sea level (RSL), inferred from stacks of peaty, muddy, and sandy sediment beneath tidal wetlands, remain the most convincing evidence of magnitude 8 to 9 plate-boundary earthquakes at the Cascadia subduction zone of British Columbia, Washington, Oregon, and northern California. Where most dramatic, the sudden rises in RSL are recorded by tidal-flat or low-marsh mud grading upward into high marsh peat or spruce-wetland peat abruptly capped by tsunami-deposited sand and tidal-flat mud. Despite the dramatic stratigraphy at some sites, Cascadia great-earthquake history is sketchy because its chief archive—stratigraphic records of Holocene RSL rise beneath estuarine wetlands—has not been thoroughly scrutinized. Some unanswered questions: Which estuarine sequences of peat and mud record regional coastal subsidence during great earthquakes and which record localized effects of storms, river floods, and slowly decelerating sea-level rise? Can estuarine stratigraphy record the full amount of coastal subsidence during great earthquakes or does rapid postseismic uplift make subsidence estimates minimum values? Do centimeters of coastal subsidence over decades precede coseismic subsidence of 1-2 m? What was the coastal extent of deformation along the 1200-km-long subduction zone during each great earthquake? How far inland did deformation zones of great earthquakes extend and how did this affect the strength of shaking?

Paleogeodetic studies of tidal microfossils might provide answers to such questions. So far, microfossils (foraminifers, diatoms, and pollen) have been studied qualitatively at less than a quarter of Cascadia estuaries, and only semi-quantitative methods with large uncertainties (±0.5-1.0 m) have been used to estimate amounts of subsidence during less than four great earthquakes at any site. Initial results of transfer-function analysis of foraminiferal and diatom assemblages from a marsh core at Alsea Bay on the central Oregon coast illustrate problems and potential of paleogeodesy for addressing questions of Cascadia earthquake history.